Genomic instability is a common feature that can be found in most cancer types. Protection of chromosome ends is provided by nucleoprotein structures called telomeres. Telomeric DNA is composed of a highly conserved 3' single-stranded overhang. When unprotected, this overhang initiates lethal DNA damage responses that can cause end-to-end fusion, degradation, and cell cycle arrest. Besides cancer, telomere dysfunctions can lead to various human diseases and degenerative aging. The proposed studies will use standard biochemical, biophysical, and structural techniques to investigate how telomere end protection has evolved. Strong evidence suggests that Cdc13, Stn1, and Ten1 proteins form a trimeric complex that protect telomeric ends in yeast. However, a direct interaction between these three proteins has never been observed, and this has lead to contradicting views on how these proteins function. Recently, similar complexes have been found in humans that do not appear to play a role in the capping of telomeric DNA. We wish to evolutionary track the roles of Cdc13, Stn1, and Ten1 through different species, in order to better understand what functions certain complexes play in telomere end protection. In Aim 1, we will elucidate for the first time in vitro if Cdc13, Stn1, and Ten1 interact with each other. The structure of this complex in several species will be determined to elucidate how these proteins play a role in end-protection. Aim 2 will track how the DNA binding protein Cdc13, and its subunits Stn1 and Ten1, has evolved in their recognition of telomeric DNA. A large sample of Cdc13 proteins will be tested for their ability to bind telomeric DNA. The collected data will provide insight on how telomeric end protection from yeast had evolved into complexes with more genomic wide functions found in humans. The biochemical and structural nature of this work will provide evidence for the first time if Cdc13, Stn1, and Ten1 interact. We will also elucidate how the recognition of telomeric DNA has changed over time from different species. This in vitro data will provide a novel connection of how end-protection has evolved.